The invention relates to the field of vestibular stimulation, and in particular to a galvanic vestibular stimulation system and method.
Galvanic vestibular stimulation has proven to be a valuable technique for studying the role played by vestibular information in the control of stance and balance. With this technique, small-amplitude galvanic current is delivered transcutaneously to the vestibular afferents that lie directly below the mastoid bones. This serves to modulate the continuous firing level of the peripheral vestibular afferents. Specifically, cathodal (negative) currents increase the firing rate of vestibular afferents, whereas anodal (positive) currents decrease the firing rate of vestibular afferents. Thus, constant bipolar galvanic current produces a tonic vestibular asymmetry. This effect causes a standing subject to lean in different directions depending on the polarity of the current and the direction of the subject's head. In general, a subject will tend to lean toward the anodal stimulus (in the direction of the vestibular apparatus with reduced afferent activity levels) and/or away from the cathodal stimulus (away from the vestibular apparatus with increased afferent activity levels).
A considerable number of studies have examined the body-sway response to constant galvanic stimulation of the vestibular system. One study, for instance, used monopolar monaural constant galvanic stimulation and demonstrated that the amplitude of the body-sway response increases linearly with increasing stimulus current (from 0.2 mA to 1.0 mA). Another study used bipolar binaural constant galvanic stimulation and showed that the direction of the evoked sway is approximately in the direction of the intermastoid line. Thus, with bipolar binaural constant galvanic stimulation, lateral sway is produced if a subject's head is facing forward, whereas anteroposterior sway is produced if a subject's head is turned to the left or right (over the left or right shoulder).
A limited number of studies have shown that the application of sinusoidally varying bipolar galvanic currents to the vestibular system can lead to sinusoidally-varying postural sway. With sinusoidal galvanic stimulation, as with constant galvanic stimulation, the body tends to sway towards the positive stimulus and away from the negative stimulus. For low-frequency stimulation, the frequency of the evoked body sway matches the frequency of the stimulus, whereas the amplitude of the evoked body sway varies from subject to subject.